1. Technical Field
The embodiments herein generally relate to electrical circuits for gain control, and, more particularly, to automatic gain control (AGC) loop circuits.
2. Description of the Related Art
In a wireless communication environment, a tuner is provided in receivers such as radios and televisions to tune to a channel selected by a user. The tuner typically comprises a mixer that mixes the received radio frequency (RF) signal with a local oscillator (LO) signal having a frequency corresponding to a selected channel. The output of the mixer is a predetermined intermediate frequency signal. In such cases, a radio signal received by the receiver varies with time as the receiver's location changes. Such changes may lead to compression in the tuner or loss of sensitivity.
As a result, maintaining a correct level of the radio signal is important for proper receiver design. First, the input signal must be at the correct level before entering an analog-to-digital converter (ADC) to reduce the dynamic range of the ADC. Dynamic range refers to the ratio of a maximum level of a parameter, such as voltage or frequency, to the minimum detectable value of the parameter. If the input signal level is either too big or too small then the ADC output signal will be either clipped or rounded.
Generally, both clipping and rounding of the output signal will significantly degrade the signal quality and performance of a system. Second, the input signal level before each amplifier should be at the correct level to keep the amplifiers' linearity. Mobile digital television standards such as Digital Video Broadcasting-Handheld (DVB-H), Terrestrial Integrated Services Digital Broadcasting (ISDB-T), and MediaFLO™ mobile multimedia multicast system available from Qualcomm, Inc., California, USA use Orthogonal Frequency Division Multiplexing (OFDM) which is assumed as an effective digital modulation scheme for transmitting data at high rate in many wireless applications, where the radio signals are split into multiple smaller sub-signals that are then transmitted simultaneously at different frequencies to the receiver. OFDM has many advantages such as high spectrum efficiency, robustness at multi-path channels and high frequency diversity gain. On the other hand, OFDM also has disadvantages such as a high peak to average power ratio (PAPR) (e.g., usually 12˜14 dB) and high sensitivity to inter carrier interference (ICI).
Terrestrial television and radio broadcast networks tend to have large blocker signals covering broad service areas. In such cases, the receiver typically has to be very linear. In other words, the output generally must have a linear relationship with the input, to maintain the performance of the receiver. Blocker signals are unwanted signals in frequency channels outside the wanted channel that disturb the reception of the wanted signals. This happens due to the fact that the blockers generate large signals within the receiver path. Generally, it is difficult to achieve optimal performance for a wide frequency range in the presence of blocker signals which cannot be filtered.